1. Field of the Invention
The present invention relates to a current perpendicular to the planes (CPP) sensor with a highly conductive cap structure and, more particularly, to such a cap structure which includes ruthenium (Ru), rhodium (Rh) or gold (Au) and a method of making.
2. Description of the Related Art
The heart of a computer is a magnetic disk drive which includes a rotating magnetic disk, a slider that has read and write heads, a suspension arm above the rotating disk and an actuator arm that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. When the disk is not rotating the actuator arm parks the suspension arm and slider on a ramp. When the disk rotates and the slider is positioned by the actuator arm above the disk, air is swirled by the rotating disk adjacent an air bearing surface (ABS) of the slider causing the slider to ride on an air bearing a slight distance from the surface of the rotating disk. The ABS is an exposed surface of the slider and the write and read heads that faces the rotating disk. When the slider rides on the air bearing, the actuator arm positions the write and read heads over the selected circular tracks on the rotating disk where field signals are written and read by the write and read heads. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
An exemplary high performance read head employs a current perpendicular to the planes (CPP) sensor, such as a magnetic tunnel junction (MTJ) sensor, for sensing the magnetic field signals from the rotating magnetic disk. The MTJ sensor includes an insulative tunneling or barrier spacer layer sandwiched between a ferromagnetic pinned layer and a ferromagnetic free layer. An antiferromagnetic pinning layer interfaces the pinned layer for pinning the magnetic moment of the pinned layer 90° to the air bearing surface (ABS). The MTJ sensor is located between ferromagnetic first and second shield layers. First and second leads, which may be the first and second shield layers, are connected to the MTJ sensor for conducting a tunneling current therethrough. The tunneling current is conducted perpendicular to the major film planes (CPP) of the sensor as contrasted to a spin valve sensor where the sense current is conducted parallel to or, otherwise stated, conducted in the planes of the major thin film planes (CIP) of the spin valve sensor. Another type of CPP sensor employs a nonmagnetic conductive material for the spacer layer instead of an insulation material. A magnetic moment of the free layer is free to rotate upwardly and downwardly with respect to the ABS from a quiescent or zero bias point position in response to positive and negative magnetic signal fields from the rotating magnetic disk. The quiescent position of the magnetic moment of the free layer, which is parallel to the ABS, occurs when the tunneling current is conducted through the sensor without magnetic field signals from the rotating magnetic disk.
When the magnetic moments of the pinned and free layers are parallel with respect to one another the resistance of the MTJ sensor to the tunneling current (IT) is at a minimum and when the magnetic moments are antiparallel the resistance of the MTJ sensor to the tunneling current is at a maximum. Changes in resistance of the sensor is a function of cos θ, where θ is the angle between the magnetic moments of the pinned and free layers. When the tunneling current (IT) is conducted through the sensor, resistance changes, due to field signals from the rotating magnetic disk, cause potential changes that are detected and processed as playback signals. The sensitivity of the sensor is quantified as magnetoresistive coefficient dr/R where dr is the change in resistance of the sensor from minimum resistance (magnetic moments of free and pinned layers parallel) to maximum resistance (magnetic moments of the free and pinned layers antiparallel) and R is the resistance of the MTJ sensor at minimum resistance. The dr/R of a MTJ sensor can be on the order of 40% as compared to 15% for a spin valve sensor.
MTJ sensors are classified as either a top sensor or a bottom sensor. In a bottom sensor the pinning layer is closer to the first shield layer than the second shield layer and in a top sensor the pinning layer is closer to the second shield layer than to the first shield layer. In either type of sensor the first and second shield layers may engage the bottom and the top respectively of the sensor so that the first and second shield layers serve as leads for conducting the tunneling current through the sensor perpendicular to the major planes of the layers of the sensor. The sensor has first and second side surfaces which are normal to the ABS. First and second hard bias layers are adjacent the first and second side surfaces respectively for longitudinally biasing the free layer in a single domain state. This longitudinal biasing also maintains the magnetic moment of the free layer parallel to the ABS when the read head is in the quiescent condition.
In CIP devices, it is desired that a maximum amount of the current flow in the free and pinned layers interfacing the spacer layer and that current shunting through seed layers and capping layers of the sensor be minimized or eliminated. In MTJ devices, however, these layers need to be very conductive for maximum efficiency. Because the sense current in MTJ devices flows perpendicular to the thin film planes the conductance of the bottom and top layers of the sense layer is critical for optimum and reliable operation of these devices. In today's MTJ devices tantalum is used as a capping layer as well as a lead layer which is a source of concern for high resistivity and process complexity. Because of a high affinity to oxygen, the tantalum layer will always be a concern for reliable performance. During atmosphere exposure, the tantalum layer oxidizes and resistivity increases by many orders of magnitude. This oxidized layer must be milled away to ensure good electrical conductance. The milling process is not precise and variation in the amount of milled material deteriorates the gap control. For future MTJ devices this can be critical for accurate control of the read gap.